Positive displacement pumps have a suction side or chamber into which fluid flows and a discharge side or chamber out of which fluid is expelled. The overall volume in the positive displacement pump is constant, while the volumes of the suction side and the discharge side have an inverse relationship with respect to one another. One example of a positive displacement pump is a diaphragm pump, also referred to as a membrane pump, that uses the reciprocating action of a flexible diaphragm (e.g., an elastomeric membrane) and suitable valves on either side of the diaphragm to pump the fluid. The diaphragm is sealed such that one side of the pump receives and discharges the fluid to be pumped, e.g., a liquid, gas, multiphase fluid or slurry, and the other side of the pump receives and discharges a second fluid referred to as the driving fluid. The flexing of the diaphragm causes the volume of the chambers to increase and decrease. As the volume of a chamber increases, pressure in the chamber decreases and fluid is drawn in. As the volume of the chamber decreases, pressure in the chamber increases and fluid is forced out. Check valves are used to prevent reverse flow of the fluid.
Another example of a positive displacement pump is a reciprocating type piston pump that uses the reciprocating action of a piston to similarly receive and discharge fluid.
Positive displacement pumps are known for use in subsea pumping operations, e.g., for pumping drilling fluids, production fluids and slurries. In conventional deepwater drilling, drilling operators work through a blowout preventer at the sea floor to drill deepwater wells within a steel riser extending from a drillship. Drilling mud is pumped down a rotating drill pipe to lubricate the drill bit and carry the rock cuttings back up to the drillship within the riser. The drilling mud's weight causes the riser's inside pressure can be nearly twice that of the deep ocean. In subsea mudlift drilling (SMD), drilling mud flows downwards inside the rotating drill pipe to the drill bit, while a seawater-powered fluid driven positive displacement pump (FDPDP) above the blowout preventer (BOP) circulates drilling mud, also referred to as mud, and cuttings back to the drillship through a pipe outside the riser. The riser is filled with a seawater-like fluid, drilling mud or well bore fluid of similar density so the riser pressure adjusted to accommodate the challenge of dealing with fluctuating pressures from the fields under the sea floor. The main goal of the FDPDP is to regulate the mudline mud pressure such that bottom-hole pressure stays within the desired drilling pressure window, i.e., between the formation pore pressure and the formation fracture pressure.
The FDPDP is specially designed for this application. The FDPDP returns mud at the mudline via a separate path outside the drill pipe therefore lowering and/or eliminating mud-return riser friction and mud hydrostatic pressure to regulate downhole pressure, i.e., at downhole location, also referred to as bottom hole pressure (BHP), during drilling conditions to stay within the desired drilling pressure window. The drillship sends seawater down a pipe via a seawater pump to the FDPDP, also referred to as the pump, where it pulses over a diaphragm in the pump vessel, pushing the mud back up its own pipe to the drillship. The seawater, endless in supply and harmless to the environment, is discharged back to the sea, controlled by a fill choke. The pump can be designed to be installed on the drilling stack. The operation seawater depth can range between 4,000 ft and 10,000 ft. This pressure can be much lower than the mudline shut in pressure due to the mud weight. The fluid driven positive displacement pump typically includes a plurality of pressure vessels connected in parallel, each pressure vessel having a first and a second chamber separated by a diaphragm or a piston.
U.S. Pat. No. 6,505,691, entitled Subsea Mud Pump and Control System (Judge et al.), discloses a subsea pump having a plurality of pressure vessels each having two chambers with a separating diaphragm between the chambers. Each of the two chambers is hydraulically connected to receive and discharge a hydraulic fluid and a drilling fluid, respectively. The diaphragm moves in response to a pressure differential between the chambers. A hydraulic power supply is arranged to pump the hydraulic fluid to one chamber of each pressure vessel. A valve assembly is coupled to these chambers and to the hydraulic power supply. The volume of each of the chambers is measured. A valve controller connected to the valve assembly is arranged to control the rate and timing of the flow of the hydraulic fluid into and out of the chambers in response to the volume measurements. The valve controller is configured to approximate a substantially constant pump inlet pressure, a substantially constant pump discharge pressure, and/or a substantially constant volume of the first chambers.
The current method to control the timing of positive displacement pumps is the use of valve controllers with control software. The software is configured using pumping events and timers, based on empirical data, and then tuned to adjust to specific parameter changes. Thus, the control is set up empirically based on estimated conditions at a given point in time, and not in an adaptive manner. Any changes require manually changing the control software tuning parameters that control the pump hardware. Thus, current methods are not able to achieve constant pressure at varying operational conditions.
Maintaining a substantially constant pump inlet pressure is important to help ensure stable pressure within an operating window. In the example of subsea pumping of drilling fluid, a BHP that is above the operating window may result in fracturing of the formation. A BHP that is below the operating window may result in a collapse of the well or influx from the wellbore if BHP as there is no casing in place yet, depending on the formation pressure. Mud having a target mud weight is pumped by the FDPDP to regulate the BHP. Current control schemes for controlling the FDPDP are not able to adequately achieve the target operating window within a consistent, narrow range of error. Thus, there are large variations and spikes in FDPDP inlet pressure and therefore also BHP. This further has the overall effect of slowing drilling since changes in pump speed are made very gradually.
It would be desirable to have a simpler, more adaptive and more reliable method for controlling fluid driven positive displacement pumps during subsea pumping operation that would avoid the aforementioned problems.